FIG. 2 illustrates an example of a configuration of a fuse element reading circuit. FIG. 3 illustrates the resistances and frequencies of electric fuse elements. An electric fuse element 211 has a resistance that differs in accordance with whether the electric fuse element 211 is in a blown state or an unblown state. The electric fuse element 211 in the unblown state has a small resistance, whereas the electric fuse element 211 in the blown state has a large resistance. In FIG. 3, a frequency distribution 301 represents the frequency of the electric fuse element 211 in the unblown state, and a frequency distribution 302 represents the frequency of the electric fuse element 211 in the blown state.
At the time of reading, a control node SEN is at a high level, and a read voltage output circuit unit 201 outputs a read voltage V1 corresponding to the resistance of the electric fuse element 211. The read voltage V1 is low when the electric fuse element 211 is in the unblown state, whereas the read voltage V1 is high when the electric fuse element 211 is in the blown state. A reference voltage output circuit unit 204 outputs a reference voltage V3 by resistance division.
A selection circuit unit 202 selects one of read voltages V1 of a plurality of read voltage output circuit units 201, and outputs the selected read voltage V1 as a read voltage V2. A voltage comparison circuit unit 203 compares the read voltage V2 with the reference voltage V3, and outputs a voltage V4, which represents a comparison result. A latch circuit unit 205 latches the voltage V4, and outputs a voltage OUT.
The electric fuse element 211 is capable of storing data in accordance with whether the electric fuse element 211 is in the blown state or the unblown state. If the resistance of the electric fuse element 211 is smaller than a threshold resistance 303 (see FIG. 3), it is determined that the electric fuse element 211 is in the unblown state. Meanwhile, if the resistance of the electric fuse element 211 is greater than the threshold resistance 303, it is determined that the electric fuse element 211 is in the blown state. The read voltage V2 is a voltage corresponding to the resistance of the electric fuse element 211. The reference voltage V3 is a voltage corresponding to the threshold resistance 303. If the read voltage V2 is lower than the reference voltage V3, the voltage comparison circuit unit 203 determines that the electric fuse element 211 is in the unblown state. Meanwhile, if the read voltage V2 is higher than the reference voltage V3, the voltage comparison circuit unit 203 determines that the electric fuse element 211 is in the blown state.
As described above, it is necessary for the electric fuse element 211 in the unblown state to have a resistance smaller than the threshold resistance 303, and it is necessary for the electric fuse element 211 in the blown state to have a resistance greater than the threshold resistance 303. To this end, a test for the resistance of the electric fuse element 211 is carried out. When the electric fuse element 211 is in the unblown state, if the read voltage V2 is lower than the reference voltage V3, the test is successful. When the electric fuse element 211 is in the blown state, if the read voltage V2 is higher than the reference voltage V3, the test is successful.
However, as depicted in FIG. 3, a variation in the resistance of the electric fuse element 211 may occur for various reasons. The electric fuse element 211 having a resistance within a region 304 or 305 passes the test. However, since the regions 304 and 305 are in the vicinity of the threshold resistance 303, erroneous determination may be made in the case of reading data of the electric fuse element 211. Namely, a sufficient reading margin is not ensured.
In addition, the semiconductor integrated circuit described below is described in Japanese Unexamined Patent Application Publication No. 2006-310457, the entirety of which is hereby incorporated by reference. The semiconductor integrated circuit includes a first fuse element, which stores data in accordance with a difference in resistance, depending on the presence or absence of electric writing, and from which data is read in accordance with the resistance when a voltage is applied across the ends of the first fuse element, and a second fuse element, which has a characteristic equivalent to that of the first fuse element and which is used for read trim control for determining a read condition relative to the first fuse element. The semiconductor integrated circuit further includes a trim value automatic control circuit unit, which reads data of the second fuse element by applying a constant voltage across the ends of the second fuse element before reading the data of the first fuse element, and which outputs a trim value specifying a desired read condition relative to the first fuse element in accordance with a read result, and a fuse data read circuit unit, which applies a constant voltage across the ends of the first fuse element at the time of reading the data of the first fuse element, and which reads the data in accordance with the trim value supplied from the read trim value automatic control circuit unit.
In addition, the resistance-change-type fuse circuit described below is described in Japanese Unexamined Patent Application Publication No. 2007-299926, the entirety of which is hereby incorporated by reference. The resistance-change-type fuse circuit includes a plurality of polysilicon fuses, which are made of polysilicon and which cause irreversible changes in resistance by causing a current to flow, a plurality of programming transistors, which are arranged in association with the plurality of fuses and which switch between causing and not causing currents to flow through the corresponding fuses, so as to cause changes in the resistance, and a dummy fuse group that includes a plurality of dummy fuses that have the same electric characteristic as that of the plurality of polysilicon fuses, and that have a resistance that is 1/n times the resistance of the plurality of polysilicon fuses. The resistance-change-type fuse circuit further includes a dummy transistor circuit unit that includes at least one dummy transistor that has a conductance that is 1/n times the conductance of the plurality of programming transistors, and that has a gate and drain connected to each other, and a current mirror circuit unit that includes the programming transistor and the dummy transistor, and that causes a current that is n times the current flowing in the dummy fuse group to flow in the plurality of polysilicon fuses.